A condenser microphone includes a diaphragm configured to vibrate in response to acoustic waves from a sound source and a fixed electrode constituting a capacitor with the diaphragm. The capacitance of the capacitor varies in response to the vibration of the diaphragm. The condenser microphone generates electrical signals corresponding to the variation in the capacitance of the capacitor. The generated electrical signals are output to, for example, a speaker connected to the condenser microphone.
Condenser microphones can be set to have various directionalities. One of the directionalities is unidirectionality. A unidirectional condenser microphone is used for sound collection in a specific direction (for example, the front direction).
The unidirectional condenser microphone includes a unidirectional condenser microphone unit. The unidirectional condenser microphone unit includes an acoustic resistor for achieving unidirectionality, in addition to the diaphragm and the fixed electrode.
FIG. 7 is a cross-sectional front view illustrating a conventional unidirectional condenser microphone unit. A unidirectional condenser microphone unit (hereinafter referred to as “unit”) 101 includes a unit case 110, a diaphragm 120, a diaphragm holder 130, a fixed electrode 140, an insulating base 150, an air chamber 160, an acoustic resistor 170, and a metal mesh 180.
The unit case 110 accommodates the diaphragm 120, the diaphragm holder 130, the fixed electrode 140, the insulating base 150, the acoustic resistor 170, and the metal mesh 180. The unit case 110 has shape of a hollow cylinder with a bottom end. The unit case 110 is a press-molded product composed of metal, such as aluminum. The unit case 110 includes multiple acoustic-wave entering holes 110h introducing acoustic waves from a sound source into the unit 101. The multiple acoustic-wave entering holes 110h are disposed in a bottom face side (the upper side of FIG. 7) of the unit case 110. In the description below, the bottom face side (the upper side of FIG. 7) of the unit 101 having a shape of a hollow cylinder with a bottom end is referred to as the “front” of the unit 101, and an open end side (the lower side of FIG. 7) of the unit 101 is referred to as the “rear” of the unit 101.
The diaphragm 120 is a thin film having a circular shape in plan view. The diaphragm 120 is composed of synthetic resin, for example. The diaphragm 120 is stretched on the diaphragm holder 130 at predetermined tension. The diaphragm holder 130 has a shape of a ring in plan view.
FIG. 8 is a plan view illustrating the fixed electrode 140 of the conventional unit 101. The fixed electrode 140 has a shape of a disc in plan view. The fixed electrode 140 is composed of metal. The fixed electrode 140 has multiple sound holes 140h. The multiple sound holes 140h are disposed over the entire face of the fixed electrode 140.
Referring now back to FIG. 7, the fixed electrode 140 faces the diaphragm 120 with a spacer (not shown) disposed therebetween, and constitutes a capacitor with the diaphragm 120. An air layer having a thickness equivalent to the thickness of the spacer is formed between the diaphragm 120 and the fixed electrode 140.
FIG. 9 is a plan view illustrating the insulating base 150 of the conventional unit 101. The insulating base 150 has a shape of a disc in plan view. The insulating base 150 is composed of synthetic resin, for example. The insulating base 150 has a communication hole 150h, a depression 151, and a support 152. Acoustic waves from the sound source pass through the communication hole 150h. The depression 151 faces the fixed electrode 140 and defines the air chamber 160 together with the fixed electrode 140. The support 152 supports the fixed electrode 140.
As shown in FIG. 7, the acoustic resistor 170 covers the communication hole 150h from the rear side of the communication hole 150h. The acoustic resistor 170 is composed of a material, such as nonwoven fabric, sponge, or felt (for example, refer to Japanese Patent Publication No. 5484882). The acoustic resistor 170 functions as an acoustic resistor reducing the velocity of acoustic waves passing from the sound source through the acoustic resistor 170.
The metal mesh 180 has a shape of a disc in plan view. The metal mesh 180 is composed of metal. The metal mesh 180 is disposed between the unit case 110 and the diaphragm holder 130 and covers the acoustic-wave entering holes 110h from the rear side of the acoustic-wave entering holes 110h. The metal mesh 180 prevents intrusion of foreign objects from outside the unit case 110 into the unit case 110.
The metal mesh 180, the diaphragm holder 130 (diaphragm 120), the fixed electrode 140, the insulating base 150, and the acoustic resistor 170 are accommodated in the unit case 110, in this order from the opening of the unit case 110. The acoustic resistor 170 accommodated in the unit case 110 is disposed at the opening of the unit case 110 so as to cover the opening of the unit case 110 from the interior of the unit case 110.
The fixed electrode 140 is supported by the support 152 of the insulating base 150 inside the unit case 110. The sound holes 140h of the fixed electrode 140 face the depression 151 in the insulating base 150. The air chamber 160 is formed between the fixed electrode 140 and the depression 151.
The air chamber 160 adjusts the level of vibration of the diaphragm 120 in accordance with the volume of the air chamber 160. The air chamber 160 is in communication with the sound holes 140h in the fixed electrode 140 and the communication hole 150h in the insulating base 150.
The acoustic resistor 170 is fixed to the rear (back) face of the insulating base 150 by curling of the rear edge of the unit case 110. As a result of the curling, a curled portion 111 is formed in the rear edge of the unit case 110.
The operation of the unidirectional condenser microphone unit will now be described.
In the description below, among the acoustic waves from a sound source, the acoustic waves entering from the acoustic-wave entering holes 110h into the unit 101 and reach the front face of the diaphragm 120 are referred to as “front-face acoustic waves,” and the acoustic waves entering from the communication hole 150h into the unit 101 and reach the back face of the diaphragm 120 are referred to as “back-face acoustic waves.”
The operation of the unidirectional condenser microphone with a sound source disposed in front of the unidirectional condenser microphone (in front of the unit 101) will now be described.
The front-face acoustic waves reach the diaphragm 120 from the acoustic-wave entering holes 110h through the metal mesh 180. On the other hand, the back-face acoustic waves reach the diaphragm 120 through the acoustic resistor 170. As described above, the acoustic resistor 170 functions as an acoustic resistor. Thus, the velocity of the acoustic waves traveling through the acoustic resistor 170 is reduced by the acoustic resistor 170. The acoustic waves decelerated inside the acoustic resistor 170 reach the diaphragm 120 through the communication hole 150h, the air chamber 160, and the sound holes 140h. 
The distances between the sound source and the respective acoustic-wave entering holes 110h are each smaller than the distance between the sound source and the acoustic resistor 170. Thus, the front-face acoustic waves reach the diaphragm 120 before the back-face acoustic waves. The back-face acoustic waves reach the diaphragm 120 after the front-face acoustic waves.
The diaphragm 120 is configured to vibrate in response to the acoustic waves reaching the diaphragm 120. The capacitance of the capacitor constituted by the diaphragm 120 and the fixed electrode 140 varies in response to the vibration of the diaphragm 120. The unit 101 generates an electrical signal corresponding to the variation in the capacitance. As described above the acoustic waves from the sound source in front of the unidirectional condenser microphone are collected by the unidirectional condenser microphone (unit 101).
The operation of the unidirectional condenser microphone with a sound source disposed behind the unidirectional condenser microphone (behind the unit 101) will now be described.
The front-face acoustic waves reach the diaphragm 120 from the acoustic-wave entering holes 110h through the metal mesh 180. On the other hand, the back-face acoustic waves reach the diaphragm 120 through the acoustic resistor 170. The velocity of the acoustic waves traveling through the acoustic resistor 170 is reduced. The acoustic waves decelerated inside the acoustic resistor 170 reach the diaphragm 120 through the communication hole 150h, the air chamber 160, and the sound holes 140h. 
The distances between the sound source and the respective acoustic-wave entering holes 110h are each larger than the distance between the sound source and the acoustic resistor 170. The acoustic resistance of the acoustic resistor 170 is designed such that the timing of the front-face acoustic waves reaching the diaphragm 120 matches the timing of the back-face acoustic waves reaching the diaphragm 120. Thus, the timing of the front-face acoustic waves reaching the diaphragm 120 matches the timing of the back-face acoustic waves reaching the diaphragm 120.
The diaphragm 120 does not vibrate when the timing of the front-face acoustic waves reaching the front face of the diaphragm 120 matches the timing of the back-face acoustic waves reaching the back face of the diaphragm 120. That is, the unit 101 does not generate an electrical signal because the capacitance of the capacitor does not vary. In other words, the sound from the sound source behind the unidirectional condenser microphone is not collected by the unidirectional condenser microphone (unit 101).
As described above, the unit 101 collects sound from the sound source in front of the unit 101 but does not collect sound from the sound source behind the unit 101. That is, the directionality of the unit 101 is unidirectionality.
Since the acoustic resistor 170 is composed of a material, such as sponge, the acoustic resistance may vary due to the external environment, including humidity. For example, the acoustic resistor 170 absorbs moisture and expands in volume in a high humidity environment due to sweating of a user. The acoustic resistance of the acoustic resistor 170 increases due to the expansion in volume of the acoustic resistor 170. As a result, the degree of deceleration of the back-face acoustic waves passing the acoustic resistor 170 varies. Thus, the timing of the back-face acoustic waves from the sound source behind the unidirectional condenser microphone (behind the unit 101) reaching the diaphragm 120 does not match the timing of the front-face acoustic waves from the same sound source reaching the diaphragm 120. As a result, the diaphragm 120 vibrates. That is, the unit 101 collects sound from the sound source behind the unidirectional condenser microphone. In other words, the directionality of the unit 101 is affected by the variation in the acoustic resistance of the acoustic resistor 170.
An object of the present invention is to solve the problems described above and to provide a unidirectional condenser microphone unit and a unidirectional condenser microphone having directionality unaffected by the external environment, and a method of manufacturing the unidirectional condenser microphone.